OPV plug failed open at 120psi

[tbone1004]

the OPV's are basically identical to the spring and seat setup on an unbalanced second stage. When you rebuild, you back the adjustment screw all the way out, take the spring out, swap in a new seat, and reassemble/adjust

 

[Akimbo]

General information for readers who may not fully understand this thread:
An OPV (Over Pressure Valve) is more accurately a pressure relief valve. Functionally they are no different than the relief valves on water heaters. Good pressure system design calls for a relief valve when there is a reasonable possibility that a system can be overpressurized and a failure could harm people or property. That is the theory behind blow out plugs in Scuba cylinder valves.

The most common second stage Scuba regulators use a "downstream" design, which act as pressure relief valves. Therefore it isn't necessary to install OPVs on most Scuba regulators. There are cases where the potential failure of the OPV or relief valve increases diver risk more than a hose failure. As demonstrated by this thread, relief valves can fail open.

There is a continuing debate on how to manage the risk associated with them. A emergency bailout bottle connected to a FFM (Full Face Mask) or commercial diving helmet is a classic example. The HP bailout is basically a Scuba tank with a first stage attached. The IP (Intermediate Pressure) hose connects to a normally-closed manual valve on a manifold. The debate is if the risk of a relief valve failure is greater than the risk of the IP hose bursting from a failure of the first stage to keep the pressure below the hose bursting point. The most common first stage failures are IP creep -- the pressure slowly increases above the desired set-point for the second stage, typically +/- 135 PSI/9.3 Bar. IP creep failures are typically slow and can be found during pre-dive-day checks.

Another place where a pressure relief failure can be dangerous to divers is on a decompression chamber. This is usually managed by having a isolation valve installed between the chamber and relief valve.

 

[tbone1004]

@Akimbo I'm sure this was an oversight, but the most common second stages are downstream valves.

 

[Akimbo]

@Akimbo I'm sure this was an oversight, but the most common second stages are downstream valves.

Thanks, correction made. I re-organized that sentence while composing and missed it.

 

[tbone1004]

Thanks, correction made. I re-organized that sentence while composing and missed it.

I figured something like that happened. Happens to me every once in a while